Piezoelectric Material

Piezoelectricity, meaning ‘Electricity from Pressure' was discovered by Pierre and Jacque Curie more than 100 years ago. The so called ‘direct effect' of charge generation in a class of crystals while subjected to mechanical stress was first observed in the crystals like zinc blend, sodium chlorate, tourmaline and quartz.

The ‘inverse effect' in piezoelectricity, signifying the deformation in crystals on the application of a voltage was theoretically predicted by Lippman showing it as a consequence of a reversible thermodynamics process and later experimentally validated by Jacque Curie. Voigt did the first rigorous analysis of correlating crystal structure symmetry to piezoelectricity.

Out of the possible 32 crystal groups (based on simple lattice geometry and symmetry operation on crystal faces), it was shown that only 11 groups could have piezoelectricity. The uniqueness of these groups of piezo-crystals is their non-centro- symmetric structure, and interestingly a subgroup of such crystals always falls to a state of spontaneous polarization. The phenomenon widely known as ‘Ferro electricity' occurs due to the presence of hydrogen bond dipoles in the elements.

Over the years, guided by these characteristics, material scientists have successfully identified/developed crystals with piezoelectric property. Most of them are found unfit for engineering applications due to their high temperature sensitivity and large hysteresis loop. It is only with the invention of Barium Titanate ceramic (around 1947) that building of efficient solid-state transducer has been made possible. However, within next ten years, Barium Titanate is substituted by Lead Titanate Zirconate (commonly known as PZT) as it could operate up to a much higher temperature and possesses stronger piezoelectric effect. Similar effect has also been discovered in a polymeric crystal called Polyvinylideneflouride (PVDF) by Kawai . The recent developments in smart structural technology are broadly centered on these two piezoelectric materials.

Figure 30.1: Barium Titanate in a tetrogonal symmetry below Curie Temperature